1. The Field of the Invention
The present invention is related to energetic polymer compositions and methods for their polymerization. More particularly, the present invention is related to the polymerization of cyclic ether monomers using selected alkylating salts together with a selected alcohol as the polymerization catalyst.
2. Technical Background
It has long been desired to find efficient and effective methods of polymerization of cyclic ethers. Ring-opening polymerization of cyclic ethers is well established. For example, the polymerization of cyclic ethers with an alcohol initiator and an acid catalyst is described in U.S. Pat. No. 4,988,797. Synthesis of ABA triblock polymers and A.sub.n B star polymers from cyclic ethers is described in U.S. Pat. No. 4,952,644. In addition, the synthesis of tetrafunctional polyethers using certain tetraols is described in U.S. Pat. No. 5,098,277.
Polymers of cyclic ethers are commercially important and have been used as engineering plastics, as well as in the making of polyurethanes. Thus, effective methods of polymerization are of significant interest and of significant importance.
It has been found that certain polymers of cyclic ethers are particularly adaptable for use as binders for various compositions. Binders are generally materials which provide consistency to a composition and which bind together all of the constituent elements of a composition. Such polymerized cyclic ether formulations are particularly adaptable for use as binders in the formation of explosives and propellants. Such binders are able to carry and spatially immobilize large amounts of solid particulates, such as fuel particulates and oxidizer particulates.
One area of concern has been control of the functionality of the polymer formed from cyclic ether monomers. The functionality of the polymer is an important determinant of the polymer's chemical and physical properties. In addition, if it is desired to produce block copolymers it is clearly necessary to control the functionality of the polymer intermediates in order to control the nature of the copolymer which results.
Generally, the hydroxyl functionality of a polymer corresponds to the hydroxyl functionality of the alcohol employed in the synthesis of the polymer. Thus, if the alcohol is a diol, such as butanediol, the polyether is expected to have a hydroxyl functionality of about 2. If the alcohol is a triol, the polymer is expected to have a functionality of about 3.
Predictability of functionality is not precise in existing systems, however, because side reactions or incomplete initiation from all hydroxyl groups frequently results in a polymer which varies from the functionality of the alcohol precursor. Variation in functionality of the polymer clearly causes problems in many contexts. For example, in the use of the polymer in the formation of block copolymers variation in functionality will result in less than ideal block copolymers. Thus, it would be a significant advancement to provide methods for synthesis of polymers which would allow more precise control of the functionality.
As mentioned above, cyclic ether polymers are of interest as binders in certain types of chemical formulations. One area of significant interest is the use of cyclic ether polymers as binders for propellants and explosives. When conventional polymers are used as binders for propellants and explosives, it has been found that the binder does not contribute significantly to the energy output of the overall formulation. While the binder is consumed by the burning of the propellant or explosive, the energy added is relatively small. Accordingly, it would be beneficial to provide a binder which was also "energetic." In such a formulation, the binder itself would contribute significant energy when the energetic formulation is burned.
In order to attempt to prepare an energetic binder, it would be useful to employ monomers which include energetic functional groups. In the case of cyclic ethers, it would be useful to attach energetic functional groups to the cyclic ether molecule in order to form an energetic binder.
It is observed, however, that the addition of certain energetic functional groups to the cyclic ether monomer results in a significant slowing of the conventional polymerization reaction. Thus, polymerization efficiency is reduced. The polymerization reaction takes a longer period of time and the resulting polymer generally has a lower molecular weight than would otherwise be expected or desired.
Thus, it would be a significant advancement in the art to provide methods and compositions which overcame some of the problems otherwise encountered in the polymerization of cyclic ethers generally, and in the polymerization of cyclic ethers having attached energetic functional groups in particular. It would be an advancement in the art to provide methods for polymerizing cyclic ethers which were quicker and more efficient than known methods. It would be a related advancement in the art to provide methods for polymerizing cyclic ethers which would allow polymerization of energetic cyclic ethers. It would be a further advancement in the art to provide methods of polymerizing cyclic ethers which resulted in precise control of the functionality of the resulting polymer. It would be an advancement in the art to provide methods of polymerizing cyclic ethers which also result in precise control of molecular weight.
Such methods and compositions are disclosed and claimed herein.